1. Field of the Invention
This invention relates to a method and apparatus for measuring fine particles in a fluid, and more particularly to a method and apparatus for measuring particle characteristics such as diameter and the number of the particles by irradiating a flowing fluid with laser light and detecting the laser light that is scattered by the particles in the fluid.
2. Description of the Prior Art
A conventional method for determining particle characteristics such as the diameter and number of particles is to irradiate a measuring area with light and measure the amount of light that is transmitted, the scattering characteristics and the like.
This method is for example used to determine particulate impurities contained in purified water. However, because of the small size of the particles in the water and their sparse distribution, such measurement is difficult. Because of this, there has been employed a method whereby a high-luminance measuring area is formed by concentrating a beam of irradiating light from a laser light source or the like into a small area in order to increase the intensity of the scattered light from the particles, and receiving the light scattered by particles that pass through this area.
In a particle measuring apparatus in which particles are irradiated with laser light and the light scattered by the particles is analyzed, how the measuring portion through which the particles are caused to pass is formed is important. In the case of particles in a gas, the gas containing the particles is blown out from a nozzle and surrounded by purified gas to form the measuring area. In the case of the measurement of particles in a fluid, a measuring cell is required which maintains the fluid and causes the fluid to flow.
Because of the irradiation by the laser light, the surfaces of the measuring cell through which the laser light beam enters and exits, and the surface which receives the scattered light, need to be optically transparent. A four-sided translucent cell is used when the scattered light is received in a direction perpendicular to the direction in which the laser beam is projected, and a two-sided translucent cell is used when the scattered light is received in the forward direction of the laser beam.
With such a method, in most cases the spot of converged laser light is used as the particle detection area formed in the measuring cell to increase the scattering intensity. Also, with this system the fluid containing the particles is measured just one time when the fluid is passed through the cell, because the fluid is discharged after the measurement.
In particular, various techniques for measuring the particles are used, because the cross-sectional shape of the laser beam that forms the particle detection area, the direction of the passage of the particles and the method of setting the mask optimally are directly dependent on the capability of resolving the particle diameter of the system in which the particle diameter is determined from the intensity of the light scattered by the particles.
Previously, because the particles to be measured were relatively large, that is, up to 0.2 micrometers, the intensity of the light scattered by the particles was high, and even when particles in pure water were to be measured, there was no need to increase the concentration of the laser beam in order to distinguish the light scattered by the particles from background light by water. Therefore, there was no need to reduce the size of the mask which sets the effective diameter of the light beam constituting the particle detection area to full width at half maximum.
However, for background-light recognition and detection of particles measuring 0.1 micrometers or less, it was necessary to tightly focus the laser light forming the particle detection area into a beam having a diameter on the order of 10 micrometers and limit the length of the detection area in the direction of the optical axis to about the same, so as to thereby control the intensity of the background light. In order to realize this, a mask having a slit on the order of 10 micrometers is disposed at the imaging position of the receiving lens which collects the scattered light. However, even if the mechanical precision of the system is increased, it is not easy to constantly maintain a mask having this small slit at the optimum position in an environment subject to vibration and fluctuations in temperature. Especially for the round-the-clock on-line measuring systems, it is possible that the scattered-light imaging position on the mask surface will be moved as a result of variations in the day-to-day ambient atmospheric temperature, distortion caused by heat given off by heat sources such as the internal laser light source, and vibration.
Also, with a particle-measuring apparatus based on the light-scattering method, in order to distinguish light scattered by particles from the light scattered by the fluid at the particle detection area (hereinafter referred to as "background light"), a particle recognition system is used wherein the scattered-light intensity is converted into electrical signals, a few threshold values for the signal are set, every series of signals exceeding the threshold values is recognized as particles, and the number of particles classified according to the threshold values are counted as the particle distribution.
However, when the particle size is around 0.1 micrometers, the light scattered by the particles is weak, so that in order to extract the particle-scattered light the volume of the particle detection area has to be reduced to weaken the background light, which gives rise to a large fluctuation in the electrical signals, and it becomes difficult to clearly distinguish the signal pulses corresponding to the weak particle-scattered light from the fluctuation component, using the said particle recognition method. The cause of this lies in the use of only the signal pulse amplitude for particle recognition.